I bought Charles Platt’s Make: Electronics book from Amazon and, much to my delight, I found a circuit described there that does the pulsing/breathing thing. I then set out to buy the parts needed to make this circuit. Some contributors on the Internet have scoffed at this circuit because the transistor (the programmable unijunction transistor) is woefully old and difficult to obtain. That’s as maybe, but I bought 25 of these on eBay for $6.71 AUD (or $0.27 each if you prefer) from the eBay vendor dpi4parts. It took all of 12 days for the delivery to arrive. That didn’t seem too hard to me.
The alternative that has been suggested on the Internet (from many sources) is to use a Darlington pair. Meh … I’ll use the 2N6027, thanks.
I’ve re-rendered the circuit here in Fritzing:
Feel free to use my rendition.
And here is the exported PCB from Fritzing too.
I then transfer printed and etched the board, drilled it out and populated it.
Top view of the populated board.
Bottom view, showing my lovely soldering ;p Yah … it’s neat enough.
And here it is, powered up. When the circuit is connected to power, it takes a little while for the capacitors to “fill up” and start to cycle. Also, this circuit (as mentioned in the book) requires 9V power. If you use, say, 5V, the LED will eventually light up, but it won’t cycle. That’s got to be my only gripe about this circuit.
When it does cycle, it should look something like:
The PCB is 3.75 cm long x 2 cm wide x 1.5 cm high.
This circuit has a larger footprint than the NE555 based circuit and costs slightly more. I calculated a parts cost at $0.69 AUD for this circuit versus the $0.60 AUD for the NE555 based circuit. Go figure, I would have thought that the costs were the other way around. But then, the NE555 IC only costs $0.20 compared to the $0.27 for the PUT transistor. I guess the other factor to consider is that the NE555 based circuit runs on 5V and the components are easier to acquire.
I made a small batch (3) PCB for this, so I guess I’m going to find a use for them.